The invention relates to code-decode systems for protecting digital signal transmissions from interference of two overlayed code-decode operations known respectively as external and internal code-decodes. In accordance with the present invention, first (external) code-decode is performed by devices housed in the terminal equipment; here coding is accomplished in the transmitter terminal and decoding is accomplished in the receiver terminal. The second coding (internal) is accomplished in the intermediate transponder while the corresponding decoding is accomplished in the receiving terminal.
FIG. 1 shows an non-limiting example a possible block diagram for a system according to the present invention, in which the digital signal to be transmitted (TXSG) is encoded through an external code device (EXCD); digital signal TXSG' so obtained (with a bit rate greater than TXSG) is used to modulate through modulator MOD 1, a transmission carrier later amplified by transmitter TEQ 1 and radiated by antenna ANT1. The RF signal sent by the transmitter is received by the intermediate transponder through antenna ANT2, receiver REQ2, and demodulator DEM1, which reconstitute signal TXSG', save for transmission errors. The signal so received is once again encoded by internal coder INCD, obtaining a new digital signal TXSG" which has a bit rate even higher than TXSG'. The TXSG' signal is then used to modulate a second RF carrier by means of MOD2 modulator, later amplified by TEQ2 and radiated by antenna ANT3.
The RF signal sent by the transponder is received by the receiver terminal through ANT4 and through receiver (REQ2) and demodulation (DEM2) units, a RXSG" signal is rebuilt, coinciding, save for transmission errors, with signal TXSG". This signal is decoded by internal decoder INDC, obtaining a signal RXSG' which, save for transmission errors, coincides with signal TXSG'.
This last signal is decoded by external decoder EXDC so as to rebuild signal RXSG which, save for transmission errors, coincides with transmitted signal TXSG.
The present invention makes possible transmitter terminal-transponder and transponder-receiver terminal links that are protected by diversified coding schemes which can be chosen as a function of specific applications requirements.
The invention concerns a concatenated code-decode system for the protection against interference when digital signals are transmitted, through a regenerative transponder.
The invention may be used in digital telecommunications systems using an intermediate regenerative transponder where it is necessary to protect terminal-transponder and transponder-terminal links through diversified redundancy codes. The system is particularly suited for satellite applications, because protecting the down-link with an independent code (which is therefore independent of the on-board thermal noise values and up-link errors) improves performance for the same band and power level, when compared to systems in which the internal coding is accomplished at the transmitter terminal.
This invention may be used in a number of applications concerning digital signal transmission and, in particular, it can be used within digital telecommunications, via satellite, where the down-link is, generally, more critical than the up-link due to the limited transmitter power available in the satellite.
The use of on-board satellite encoding improves down-link characteristics using the same bandwidth and transmitted power and, leaving unchanged the up-link characteristics (in term of bit rate, power, and band). This is particularly advantageous when the satellite is called to receive a plurality of carriers modulated by different digital signals and to transmit on the down-link a single carried modulated by a single digital signal obtained by multiplexing the digital signals received through the up-link (FDMA/TDM access). Here the adoption of two distinct codes, the external code on signals transmitted through the up-link and the internal code on the down-link, optimizes the utilization of bandwidth and power in both the up- and down-links.
In particular, since, in the majority of satellite applications, the available bandwidth amplitude in up-link is equal to that available in down-link, in FDMA-TDM systems with TDM signal bit rate equal to the sum of up-link transmitted digital signal bit rates, down-link bandwidth will exceed the bandwidth of the RF signal (because all band margins typical of FDMA access are used).
Thus, by using the technique of the present invention, it is possible to increase TDM signal bit rate by adapting it to the width of the band available, when bit rate increment is used for internal coding, thus improving overall performance of the system.
The literature shows a number of applications of concatenated code-decode devices using coders and decoders which are internal and external, developed in connection with transmitter and receiver terminals applications in which the internal coding is performed within the transponder to protect the sole transponder-receiver terminal link appear to be unknown.
In the previously known devices, the internal code-decode process will be affected by the errors on the transmitter terminal-transponder link and by the contribution of transponder receiver noise: therefore the limit value acceptable (threshold) for signal to noise ratio of the repeater to receiver terminal link is higher than it is for the present invention in which such contributions do not affect internal code-decode. Therefore the attenuations introduced in the repeater-receiver terminal link of the previously-known devices have greater impact on the error margin of the decoded signal compared to when the internal code-decode is applied to the sole transponder-receiver terminal link, as in the present invention.